The invention relates to optical scanning apparatus comprising means for scanning a document to produce optical image signals and means for sensing such signals for recording purposes.
The invention includes apparatus wherein there is means which utilises the optical image signals for forming an electrostatic charge image and means for developing such charge image thereby to produce a visible reproduction of the scanned document. The invention also includes apparatus comprising transducing means for converting the optical image signals into electric signals for transmission to a recorder, e.g. a printer or visual display unit.
Document scanners are known which comprise a bodily stationary transparent document supporting plate and a scanning unit which is mounted for bodily movement parallel with and close to the plate. In use, a light image of a line across the document is focused within the unit onto the optical sensing means so that the document is scanned line by line during a scanning displacement of the unit.
Document scanners are also known which differ from those just referred to in that the scanning unit is fixed and the document supporting plate is mounted for to and fro movement relative to said unit.
With both fixed plate and movable plate apparatus the degree of fidelity with which the information presented by the scanned document is recorded depends on the accuracy with which the document or the scanning unit as the case may be is guided during the scanning operation. The reproduction is liable to be impaired even by small changes in either the direction of relative scanning movement or the spacing between the document and the image-focusing means of the scanning unit from one moment to another in the scanning operation.
The permissible tolerance depends on the quality standards to be met by the document record. For most routine document copying purposes, high precision guidance is not necessary, but in scanners to be used for the production of prints or printing plates for the graphic industry and scanners for generating electrical image information signals for onward processing, much greater accuracy is required. For optimum results, it is necessary to ensure not only that the direction of relative displacement of the document and the image-focusing system is constant but also that each line of the document, at the moment it is exposed, is in a plane coinciding with the object plane of that focusing system. Deviations from such plane not only result in imperfect focusing of the optical image but also result in variations in the illumination intensity of the document. The closer that document is to the illuminating light source, the more sensitive is the document illumination intensity to variations in the distance of the document from such source.
In practice, there is a problem in avoiding the deviations above referred to.
For apparatus comprising a movable document supporting plate, a solution to the that problem has been proposed in our co-pending European Patent Application No 90 200 670.9. The present invention provides a solution to the problem in relation to apparatus having a fixed document supporting plate.
It has been proposed to guide a scanning unit along a predetermined path by means of supporting parallel rails running along opposite sides of such path. For example, U.S. Pat. No. 4,623,937 discloses scanning apparatus wherein one such rail is in the form of a cylindrical bar which extends through a guide passage in a bearing block on one side of the unit, while the other rail provides a flat running surface for a roller mounted on the other side of the unit. The use of such rails and the mounting of the document supporting plate as proposed in that patent would make it very difficult if not impossible to comply with very low tolerance conditions in regard to the location of the document relative to the object plane of the optical focusing system.
U.S. Pat. No. 4,870,502 proposes apparatus wherein the document supporting plate and the scanning unit are supported by the same fabricated parts of the apparatus frame, these parts being in the form of profiled rails having upper and lower flanges. The upper flanges of the two rails serve to support the document supporting plate. The lower flanges serve as tracks for rollers mounted on the scanning unit. Both the path of movement of the scanning unit and the distance between the document and the object plane of the optical focusing system depend on the precision of the geometry of the profiled rails and their assembly in the apparatus. In practice it would be very difficult reliably to achieve a very high standard of accuracy in this way, especially in apparatus produced on a mass production basis.